Flare-ups are swellings around muscles and connective tissue. A flare-up occurs after injury, viral infection, overuse or spontaneous. Usually progresses from the axial regions to appendicular regions.

Swellings eventually clear up, leaving mature bone (heterotopic ossification) behind.

The formation of heterotopic ossification typically appear in the shoulder and back areas, sometimes on the scalp or head. They can be accompanied by swelling, pain, decreased movement, stiffness, warmth and redness. Never take biopsy from hard tumor in FOP child because of destructive new flare-ups.